The invention relates to a control for a load-dependently operating variable displacement pump.
Such a control is used in the case of so-called load-sensing hydraulic systems in order to control the volume flow delivered by the variable displacement pump according to the actual requirements of the consumers to be supplied. For this purpose, it is necessary to keep the pressure drop at the control edge or the control edges of the directional valve connected upstream of the at least one consumer, which valve is frequently designed as a proportional directional valve or servo valve, at a predetermined value, i.e. to set a control pressure drop or a control pressure difference which is effective at the consumer. Serving this purpose is the differential pressure control device which, by constant or continuous relieving and loading of an actuating element of the variable displacement pump adapts the volume flow of the latter to the actual consumer requirements.
The invention is based on such a load-sensing variable displacement pump control loop, which is diagrammatically represented by the prior art shown, for example, in U.S. Pat. No. 4,011,721, and in FIG. 1.
In FIG. 1, the reference symbol 2 denotes a variable displacement pump, which is driven for example by the engine of a mobile piece of equipment, such as for example a farming tractor, and feeds a consumer supply line or pump line 4, which is led to a consumer (not shown in any more detail), such as for example a power lift, a working piston of the steering, to the trailer braking system or to a control unit of the suspension hydraulics. The consumer supply line 4 is led via a directional valve 6, designed here as a 5/3-way proportional directional valve, which has an inlet connection 8, a tank connection 10, two working connections 12 and 14 as well as a connection 16 for a load-indicating line 18. The load-indicating line 18 is connected to a control surface 20 of a differential pressure controller 30, which is designed as a 3/2-way servo valve. The other control surface 22 is pressurized via a control line 24 by the pump pressure P in the consumer supply line 4 and can be displaced out of the position A shown by the force of the pressure acting on the control surface 22 against the force of a control spring 26, which adds to the force of the pressure occurring via the load-indicating line 18 at the control surface 20, into the position B, in which a tank connection 28 is blocked and a control pressure line 34 leading to a working chamber 32 of the variable displacement pump 2 is connected to a further control line 36, carrying the pump pressure P, in order to influence the variable displacement pump 2 in the sense of a controlled cutting-back of the output volume. 40 denotes a further servo valve, which can be displaced out of a position C, switching through the control pressure line 34, against the force of a restoring spring 42 by the coming into effect of the pump pressure P acting in a control line 44, into the position D, in which a control pressure line branch 34A is blocked and, via a further control line 46, a pressurization of the control pressure line 34 with the pump pressure P in the consumer supply line 4 takes place.
The proportional directional valve 6 has three main switching positions E, F and G, it being possible for the spool valve to be centred in the neutral position F by the springs 48 and 50 and displaced by suitable electric or hydraulic activation into the two other switching positions E and G, in order to effect a reversal of the admission of pressure to the working connections 12 and 14. In the switching positions E and G, the connection 16 of the load-indicating line 18 is in each case connected to the inlet connection 8, so that the pressure effective at the consumer prevails in the load-indicating line 18. In the neutral position F, the connection 16 of the load-indicating line is connected to the tank connection T, so that the load-indicating line 18 is relieved. Upon displacement of the proportional directional valve 6 out of the neutral position F, a control pressure drop occurs at the control edge of this valve, the set value of which drop is predetermined by the force of the control spring 26 of the differential pressure controller 30. In the neutral position F, in which the load-indicating line 18 is switched to the tank, the variable displacement pump 22 goes into so-called standby operation, the control spring 26 in this case determining the standby pressure. The servo valve 40 acts in the sense of a pressure limitation on the varying mechanism of the pump 2, i.e. on the working chamber 32, if the pump pressure P exceeds a maximum pressure. This limit pressure can be predetermined by the spring 42.
A pump controlled in this way is used in mobile pieces of equipment, such as for example farming tractors, bulldozers or the like, for supplying a wide variety of consumers, such as for example the steering, the power lift, the trailer braking system, the suspension hydraulics and other additional control units, which however also invariably make different requirements on the volume flow control. For instance, steering, power lift and trailer braking system in each case operate in master closed control loops, on which high requirements are made regarding speed of response and stability. The various switching functions of the suspension hydraulics additionally require the provision of relatively large volume flows in a short time. Often the entire pump output volume must be made available to the additional control units.